/**
 * The real diff surface for the git panel: parses the host's unified diff
 * text (`git diff` / `git show`) and renders it VSCode-style — per-file
 * sections with hunks (`@@ -a,b +c,d @@` headers), old/new line-number
 * gutters, and aligned context / deleted / added rows colored through the
 * DSH tokens. Untracked files produce no `git diff` output, so the caller
 * can pass the file content to render as a full-file addition instead.
 *
 * The parser is a pure function (`parseUnifiedDiff`) so the interesting
 * cases are unit-tested without a DOM.
 */
import { useMemo, useState, type ReactNode } from 'react'
import clsx from 'clsx'
import { t } from './locales.ts'
import css from './sidebar.module.css'

/** One rendered diff line. */
export interface DiffLine {
  kind: 'ctx' | 'del' | 'add' | 'meta'
  /** The line content without its diff marker ('' for the no-newline marker). */
  text: string
  /** Old-side line number (null for pure additions / metadata). */
  oldNum: number | null
  /** New-side line number (null for pure deletions / metadata). */
  newNum: number | null
}

/** One parsed hunk. */
export interface DiffHunk {
  /** The old-side start line (`-a[,b]`). */
  oldStart: number
  /** The new-side start line (`+c[,d]`). */
  newStart: number
  /** The section text after the trailing `@@` (may be empty). */
  header: string
  lines: DiffLine[]
}

/** One parsed file section of a unified diff. */
export interface DiffFile {
  /** The `---` path verbatim ('/dev/null' for a new file). */
  oldPath: string
  /** The `+++` path verbatim ('/dev/null' for a deleted file). */
  newPath: string
  /** The file changed with binary content: no hunks to draw. */
  binary: boolean
  hunks: DiffHunk[]
}

/** The parsed unified diff. */
export interface ParsedDiff {
  files: DiffFile[]
}

/** Parse the hunk header `@@ -a[,b] +c[,d] @@ section` (section may contain '@@'). */
function parseHunkHeader(line: string): { oldStart: number; newStart: number; header: string } | null {
  const match = /^@@ -(\d+)(?:,(\d+))? \+(\d+)(?:,(\d+))? @@(.*)$/.exec(line)
  if (match === null) return null
  return { oldStart: Number(match[1]), newStart: Number(match[3]), header: match[5] ?? '' }
}

/**
 * Parse `git diff --no-color` output into file sections and hunks. Rows
 * outside a file section (leading noise) and metadata rows between the
 * `diff --git`/`---`/`+++` headers and the first hunk (index lines, mode
 * changes, rename/similarity lines) are skipped; a section that never
 * reaches a hunk (a mode/rename-only change) stays hunkless so the caller
 * can still draw its path.
 */
export function parseUnifiedDiff(text: string): ParsedDiff {
  const files: DiffFile[] = []
  let current: DiffFile | null = null
  let inHunk = false
  let hunk: DiffHunk | null = null
  let oldNum = 0
  let newNum = 0
  const flushHunk = (): void => {
    if (current !== null && hunk !== null) current.hunks.push(hunk)
    hunk = null
    inHunk = false
  }
  for (const raw of text.split('\n')) {
    if (raw.startsWith('diff --git ')) {
      flushHunk()
      current = { oldPath: '', newPath: '', binary: false, hunks: [] }
      files.push(current)
      continue
    }
    if (current === null) continue
    if (raw.startsWith('Binary files ') || raw === 'GIT binary patch') {
      flushHunk()
      current.binary = true
      continue
    }
    if (raw.startsWith('--- ')) {
      flushHunk()
      current.oldPath = raw.slice(4)
      continue
    }
    if (raw.startsWith('+++ ')) {
      current.newPath = raw.slice(4)
      continue
    }
    const header = parseHunkHeader(raw)
    if (header !== null) {
      flushHunk()
      hunk = { oldStart: header.oldStart, newStart: header.newStart, header: header.header, lines: [] }
      oldNum = header.oldStart
      newNum = header.newStart
      inHunk = true
      continue
    }
    if (!inHunk || hunk === null) continue
    const marker = raw[0]
    if (marker === '\\') {
      // `\ No newline at end of file`: metadata attached to the previous row.
      hunk.lines.push({ kind: 'meta', text: raw.slice(1), oldNum: null, newNum: null })
      continue
    }
    if (marker === ' ') {
      hunk.lines.push({ kind: 'ctx', text: raw.slice(1), oldNum, newNum })
      oldNum += 1
      newNum += 1
    } else if (marker === '-') {
      hunk.lines.push({ kind: 'del', text: raw.slice(1), oldNum, newNum: null })
      oldNum += 1
    } else if (marker === '+') {
      hunk.lines.push({ kind: 'add', text: raw.slice(1), oldNum: null, newNum })
      newNum += 1
    } else {
      // Not a diff line (a hunk can never contain one): stop the hunk.
      flushHunk()
    }
  }
  flushHunk()
  return { files }
}

/** Build the untracked-file shape: one file, one hunk of pure additions. */
function untrackedFile(path: string, content: string): DiffFile {
  const lines: DiffLine[] = []
  const body = content.endsWith('\n') ? content.slice(0, -1) : content
  if (body !== '') {
    let num = 1
    for (const line of body.split('\n')) {
      lines.push({ kind: 'add', text: line, oldNum: null, newNum: num })
      num += 1
    }
  }
  return { oldPath: '/dev/null', newPath: `b/${path}`, binary: false, hunks: [{ oldStart: 0, newStart: 1, header: '', lines }] }
}

/** Strip the `a/` / `b/` prefix git puts on diff paths (not on /dev/null). */
function displayPath(path: string): string {
  if (path === '/dev/null') return path
  if (path.startsWith('a/') || path.startsWith('b/')) return path.slice(2)
  return path
}

/** The file header badge: added / deleted / renamed / binary ('' for a plain edit). */
function fileTag(file: DiffFile): string | null {
  if (file.binary) return t('diffBinary')
  if (file.oldPath === '/dev/null') return t('diffAdded')
  if (file.newPath === '/dev/null') return t('diffDeleted')
  const oldPath = displayPath(file.oldPath)
  const newPath = displayPath(file.newPath)
  if (oldPath !== newPath) return t('diffRenamed')
  return null
}

/** Cap the flattened rows like DiffBlock: head + tail, expand button between. */
const MAX_DIFF_ROWS = 500

export interface DiffViewProps {
  /** Unified diff text (`git.diff` or `git.commit-diff` payloads). */
  diff: string
  /** Untracked-file content: when present, renders as a full-file addition instead of parsing. */
  untrackedPath?: string
  untrackedContent?: string
}

export function DiffView({ diff, untrackedPath, untrackedContent }: DiffViewProps) {
  const parsed = useMemo<ParsedDiff>(() => {
    if (untrackedPath !== undefined) {
      return { files: [untrackedFile(untrackedPath, untrackedContent ?? '')] }
    }
    return parseUnifiedDiff(diff)
  }, [diff, untrackedPath, untrackedContent])
  const [expanded, setExpanded] = useState(false)

  // Flatten into display rows so the cap can slice a single list.
  const rows = useMemo(() => {
    const out: Array<{ key: string; file: DiffFile; type: 'path' | 'hunk' | 'line'; hunk?: DiffHunk; line?: DiffLine }> = []
    parsed.files.forEach((file, fileIndex) => {
      out.push({ key: `f${fileIndex}`, file, type: 'path' })
      if (file.binary) return
      file.hunks.forEach((hunk, hunkIndex) => {
        out.push({ key: `f${fileIndex}h${hunkIndex}`, file, type: 'hunk', hunk })
        hunk.lines.forEach((line, lineIndex) => {
          out.push({ key: `f${fileIndex}h${hunkIndex}l${lineIndex}`, file, type: 'line', hunk, line })
        })
      })
    })
    return out
  }, [parsed])

  const hidden = rows.length - MAX_DIFF_ROWS
  const capped = hidden > 0 && !expanded
  const headLines = Math.ceil(MAX_DIFF_ROWS / 2)
  const tailLines = MAX_DIFF_ROWS - headLines
  const head = capped ? rows.slice(0, headLines) : rows
  const tail = capped ? rows.slice(rows.length - tailLines) : []

  if (rows.length === 0) return null

  const renderRow = (row: (typeof rows)[number]): ReactNode => {
    if (row.type === 'path') {
      const tag = fileTag(row.file)
      const from = displayPath(row.file.oldPath)
      const to = displayPath(row.file.newPath)
      return (
        <div key={row.key} className={css.gitDiffFile}>
          <span className={css.gitDiffFilePath}>{to}</span>
          {from !== to && <span className={css.gitDiffFileOld}>← {from}</span>}
          {tag !== null && <span className={css.gitDiffFileTag}>{tag}</span>}
        </div>
      )
    }
    if (row.type === 'hunk') {
      const hunk = row.hunk!
      return (
        <div key={row.key} className={css.gitDiffHunk}>
          <span className={css.gitDiffHunkHeader}>@@ -{hunk.oldStart},{hunk.lines.filter(l => l.oldNum !== null).length} +{hunk.newStart},{hunk.lines.filter(l => l.newNum !== null).length} @@</span>
          {hunk.header !== '' && <span className={css.gitDiffHunkSection}>{hunk.header}</span>}
        </div>
      )
    }
    const line = row.line!
    const lineClass = line.kind === 'del' ? css.gitDiffDel : line.kind === 'add' ? css.gitDiffAdd : line.kind === 'meta' ? css.gitDiffMeta : css.gitDiffCtx
    return (
      <div key={row.key} className={clsx(css.gitDiffLine, lineClass)}>
        {line.kind === 'meta'
          ? <span className={css.gitDiffMetaText}>{line.text}</span>
          : (
            <>
              <span className={css.gitDiffNum}>{line.oldNum ?? ''}</span>
              <span className={css.gitDiffNum}>{line.newNum ?? ''}</span>
              <span className={css.gitDiffCode}>{line.text}</span>
            </>
          )}
      </div>
    )
  }

  return (
    <div className={css.gitDiff}>
      {head.map(renderRow)}
      {hidden > 0 && (
        <button type="button" className={css.gitDiffExpand} aria-expanded={expanded} onClick={() => { setExpanded(value => !value) }}>
          {expanded ? t('diffCollapse') : t('diffExpand', { count: hidden })}
        </button>
      )}
      {tail.map(renderRow)}
    </div>
  )
}